In recent years, a solid state drive (SSD) which employs a flash memory in a recording medium has been developed to reduce the cost, enlarge the capacity, and enhance the capacity. Accordingly, a storage apparatus dedicated to enterprises is enhanced in capacity by employing the SSD as a data cache in addition to general volatile memories such as a dynamic random access memory (DRAM) and a double data rate synchronous DRAM (DDR SDRAM).
Herein, the SSD has the following characteristics compared to the general volatile memory (hereinafter, referred to as DRAM/DDR) described above.
(1) Having a large capacity, and usable as a low-cost nonvolatile memory
(2) Having limits on a total amount of data in write
In other words, the SSD is limited in its life, a so-called life span. In addition, the life span of the SSD varies depending on the type of flash memory which is mounted in the SSD (a single level cell (SLC), a multi level cell (MLC)) and a mounting amount.
(3) Having an access performance lower than that of the DRAM/DDR
In general, the DRAM/DDR having a high access performance is used as a primary cache, and the SSD is used as a secondary cache.